1. Field of the Invention
The present invention relates to a recording/reproducing method, a recording/reproducing apparatus and a magneto-optical head for magneto-optical recording media. More specifically, the invention relates to a recording/reproducing method, a magneto-optical recorder and a magneto-optical head which are suitable for extra-high-density recording.
2. Description of Related Art
Recording media such as magneto-optical recording media are known as external memories for computers etc. The magneto-optical recording media can take or cover a great quantity of data such as dynamic images and voice, and are therefore used frequently as recording media in this multimedia era. It is desired that the storage capacity of magneto-optical recording media is increased.
A known method for increasing the storage capacity of a magneto-optical recording medium involves forming recording magnetic domains densely in the recording layer of the medium with a light spot having a minimized or minute diameter. For optical recording, the diameter of the light spot formed by a laser beam is represented by xcex/NA where xcex is the wavelength of the beam, and NA is the numerical aperture of the objective lens condensing the beam. It is therefore possible to minimize the light spot by either shortening the wavelength of the laser beam or increasing the numerical aperture of the objective lens. This makes it possible to form minute recording magnetic domains in the recording layer. However, if a plurality of minute recording magnetic domains exist in a reproducing light spot, there is need for a method for discriminating them and reproducing information from them.
As a method for reproducing information individually from the minute recording magnetic domains existing in a reproducing light spot, a technique for enlarging a magnetic domain and reproducing information from it as disclosed in WO98/02878 is proposed. A magneto-optical recording medium according to this technique includes a recording layer and a magnetically soft reproducing layer. It is possible to reproduce information from the recording medium by magnetically transferring the recording magnetic domains in the recording layer to the reproducing layer, and enlarging the transferred domains in the reproducing layer. This makes it possible to detect the minute recording magnetic domains inside a reproducing light spot individually as amplified signals from the reproducing layer.
It is possible to record information on a magneto-optical recording medium by irradiating the medium with a laser beam to form a heat spot based on the light intensity distribution in the light spot, then decreasing the coercive force in a high temperature region of the heat spot, and subsequently applying a recording magnetic field uniformly to the heat spot region including the high temperature region to reverse the magnetization of the high temperature region, where the coercive force has decreased, to form recording magnetic domains. In general, a magnetic coil is used to apply a magnetic field to a magneto-optical recording medium. In general, a magnetic coil is positioned on that side of a magneto-optical recording medium which is opposite to the substrate of the medium, that is to say, on the side of the medium where a recording film is formed. In general, a lens for condensing a laser beam is positioned on the side of the recording medium where the substrate lies.
If a high NA objective lens is used to minimize the light spot formed on a magneto-optical recording medium, it is necessary to shorten the distance between the lens and the adjacent surface of the recording medium because the focal distance of the lens is short. Therefore, as is the case with the magnetic coil, the high NA objective lens is positioned on the side of the recording medium where a recording film lies. The magnetic coil is constructed in such a manner that it does not intercept the light beam outgoing from the objective lens. For example, a solid immersion lens (SIL) may be used as a high NA lens. In this case, as shown in FIG. 16, a coil is wound around a SIL 10 to form a magnetic coil 104. The magnetic coil 104 can apply a magnetic field to the whole area of a light spot on a magneto-optical recording medium. The size of the recording magnetic domains formed in the recording layer of the recording medium depends on the size of the high temperature region of the heat spot. Therefore, in order to form minute recording magnetic domains, it has been necessary to minimize the heat spot. However, it has been difficult to control the size of the high temperature region of the heat spot because the size varies with the light intensity, the materials of the recording medium, etc. Consequently, it has not been easy to form smaller recording magnetic domains than the heat spot. The shape of the heat spot is reflected as the shape of recording magnetic domains. Accordingly, if the magnetic domains are overwritten, they take the form of feathers of an arrow, which is not necessarily desirable as the shape of magnetic domains for high density recording.
In view of the foregoing points, it is a first object of the present invention to provide a recording method for extra-high-density recording, and a recording/reproducing apparatus and a magneto-optical head which are suitable therefor.
It is a second object of the invention to provide a reproducing method for reproduction at high reproducing resolution of extra-high-density information recorded on a magneto-optical recording medium.
In accordance with a first aspect of the invention, a recording method is provided for recording information on a magneto-optical recording medium by applying a magnetic field to a heat spot heated by a light spot, the method comprising the steps of:
moving the light spot relative to the recording medium in such a manner that a high temperature region of the heat spot is formed outside the light spot; and
applying a magnetic field to the high temperature region of the heat spot.
The recording method according to the first aspect of the invention includes forming on the track of a magneto-optical recording medium a light spot moving relative to the medium, and forming outside the light spot a high temperature region of the heat spot produced by the light spot. This method also includes applying a recording magnetic field to the formed high temperature region to form a recording magnetic domain in the recording layer of the recording medium. In order to apply a magnetic field only to a narrow region such as the high temperature region of the heat spot, a magnetic head may be used which includes a magnetic field generating section a width of which is narrower in the direction along the track than the light spot. In this specification, the xe2x80x9chigh temperature regionxe2x80x9d conceptionally means a temperature region including the heat center, that is, the highest temperature point of the heat spot produced by a light spot which is formed on and moving relative to a magneto-optical recording medium. During information recordation, the coercive force of the recording layer of the recording medium decreases in the high temperature region so that a recording magnetic domain can be formed in this region of the recording layer. During information reproduction, the coercive force of the reproducing layer of the medium decreases in the high temperature region so that a recording magnetic domain of the recording layer can be transferred to this region of the reproducing layer. Herein, the terms xe2x80x9coutside the (a) light spotxe2x80x9d mean the outside of the region defined by the radial position where the light intensity is 1/e2 of its maximum value in the light intensity distribution is a radial direction of the light spot formed on a magneto-optical recording medium.
In the recording method according to the invention, a high temperature region of a heat spot may be formed outside the associated light spot on a rotating magneto-optical recording medium by increasing the rotational speed of the medium to increase the linear velocity of that point of the medium which is scanned with a magneto-optical head. Alternatively, if the rotational speed of the recording medium is not increased, the diameter of the light spot may be reduced further or the light intensity may be adjusted for that purpose.
The recording method according to the invention makes it possible to form rectangular recording magnetic domain in the recording layer. Under the conventional recording method, the shape of the recording magnetic domains reflects the heat spot. Consequently, if these magnetic domains are subsequently written, they take a form of feathers of an arrow, which is not necessarily desirable. In some cases, the tail of each of the domains adjoining in the direction along the track of a magneto-optical recording medium interferes with the head of the succeeding domain. Consequently, in some cases, noise is produced during information reproduction. The recording method according to the invention makes it possible to apply a narrow magnetic field only to the highest temperature region of a heat spot with a magnetic head which is narrower than the associated light spot. This makes it possible to form rectangular recording magnetic domains in the recording layer of a magneto-optical recording medium. Even if the rectangular domains adjoin in the direction along the track of the recording medium, their interference with each other decreases. This makes it possible to space the magnetic domains closely for extra-high-density-recording.
In accordance with a second aspect of the invention, a recording method is provided for recording information on a magneto-optical recording medium by applying a magnetic field to a heat spot heated by a light spot, the method comprising the steps of:
moving the light spot relative to the recording medium in such a manner that a high temperature region of the heat spot is formed off the center of the light spot in the direction along a track of the medium; and
applying a magnetic field to the inside of the high temperature region of the heat spot.
The recording method according to the second aspect of the invention includes forming on a magneto-optical recording medium a light spot moving relative to the medium, and forming a high temperature region of a heat spot at a position displaced from the center of the light spot in the direction along the track of the medium. The heat spot is produced on the basis of the light intensity distribution in the light spot. This method also includes applying a recording magnetic field to the inside of the formed high temperature region to form a recording magnetic domain in the recording layer of the recording medium. In order to apply a magnetic field only to a narrow region such as the high temperature region of the heat spot, a magnetic head may be used which includes a magnetic field generator narrower in the direction along the track than the light spot. Herein, the xe2x80x9capplication of a recording magnetic field to the inside of the high temperature regionxe2x80x9d means the application of a recording magnetic field to a region lying inside the high temperature region in the direction along the track. In this sense, the magnetic field may be applied to a region which is wider in the direction across the track than the high temperature region. Otherwise, the magnetic field may be applied to a region lying entirely inside the high temperature region.
In accordance with a third aspect of the invention, a reproducing method is provided for a magneto-optical recording medium including a recording layer and a reproducing layer, the method being characterized by the steps of:
moving a light spot relative to the recording medium in such a manner that a high temperature region of a heat spot is formed outside of the light spot on the reproducing layer; and
applying a magnetic field to the inside of the high temperature region of the heat spot to transfer a magnetic domain of the recording layer to the reproducing layer.
This method forms on a magneto-optical recording medium a light spot moving relative to the medium, and may form a high temperature region of a heat spot outside the light spot. The method also includes applying a reproducing magnetic field locally to the formed high temperature region to transfer a recording magnetic domain of the recording layer of the recording medium to the reproducing layer of the medium, from which information is reproduced. This remarkably improves the reproducing resolution for the reason stated below. The statement made below refers to MAMMOS reproduction as an example.
As shown in an upper portion of FIG. 6, a conventional reproducing method involves applying a reproducing magnetic field with a magnetic coil uniformly to a region which is wider than a light spot on a magneto-optical recording medium. This method includes forming in the light spot a high temperature region which is nearly equal in size to the recording magnetic domains formed in the recording layer of the recording medium, and applying a reproducing magnetic field to a region which is wider than the light spot to magnetically transfer a recording magnetic domain through the high temperature region to the reproducing layer, where the domain is enlarged. Therefore, the reproducing resolution depends on the size of the high temperature region of the heat spot which is based on the light intensity distribution produced by light beam radiation.
The reproducing method according to the invention includes forming a high temperature region of a heat spot outside the associated light spot, and applying a reproducing magnetic field locally to the high temperature region in the heat spot. Therefore, the reproducing resolution depends on the region to which the magnetic field is applied. Specifically, as shown in a lower portion of FIG. 6, the irradiation of the reproducing layer of a magneto-optical recording medium with a reproducing light beam forms a heat distribution in a light spot Sw on the this layer, so that a temperature region (high temperature region) where a recording magnetic domain can be transferred appears. A magnetic head 60 includes a magnetic field generator having a width W which is narrower than the light spot Sw. The magnetic head 60 applies a reproducing magnetic field to the temperature region in such a manner that the region to which the magnetic field is applied is narrow. This transfers a magnetic domain of the recording layer to the narrow region to which the magnetic field has been applied in the reproducing layer. The magnetic domain transferred to the reproducing layer enlarges in the light spot of the reproducing layer in accordance with the heat distribution produced by light radiation. Therefore, the reproducing method according to the invention makes it possible to reliably or securely extract a minute recording magnetic domain from a light spot and enlarge the domain.
In accordance with a fourth aspect of the invention, a reproducing method is provided for a magneto-optical recording medium including a recording layer and a reproducing layer, the method being characterized by the steps of:
moving a light spot relative to the recording medium in such a manner that a high temperature region of a heat spot is formed off the center of the light spot on the reproducing layer; and
applying a magnetic field to the inside of the high temperature region of the heat spot to transfer a magnetic domain of the recording layer to the reproducing layer.
The reproducing method according to the fourth aspect of the invention includes forming on a magneto-optical recording medium a light spot moving relative to the medium, and forming a high temperature region of a heat spot at a predetermined position on the reproducing layer which is displaced from the center of the light spot in the direction along the track of the recording medium. This method also includes applying a reproducing magnetic field to the inside of the high temperature region on the reproducing layer to transfer a magnetic domain of the recording layer to the region to which the magnetic field is applied in the high temperature region of the reproducing layer, from which information is read out. The reproducing layer may be a magnetic domain-enlarging/reproducing layer. In this case, on the basis of the heat distribution produced in the reproducing layer by light radiation, the magnetic domain transferred to the magnetic domain-enlarging/reproducing layer can be enlarged in the light spot so that information can be reproduced.
In accordance with a fifth aspect of the invention, a recording/reproducing apparatus is provided for a magneto-optical recording medium, the apparatus being characterized in that it comprises:
a light source for irradiating the recording medium with a light beam;
a lens for condensing the light beam from the light source to form a light spot on the recording medium;
a magnetic field source for applying a magnetic field to the recording medium; and
a device for moving the light spot and the recording medium relative to each other in such a manner that a high temperature region of a heat spot produced on the basis of the heat intensity distribution of the light spot is formed outside the light spot;
the field source and the lens being so positioned that the magnetic field is applied to the high temperature region.
The recording/reproducing apparatus according to the invention includes a device for moving a light spot and a magneto-optical recording medium relative to each other to form outside the light spot a high temperature region of a heat spot produced on the basis of the heat intensity distribution of the light spot. A magnetic field source and a lens are positioned relative to each other in such a manner that a magnetic field is applied to the high temperature region. The magnetic field source and the lens may be positioned relative to each other in such a manner that the axis of the magnetic field generator of the magnetic field source coincides with the heat center (highest temperature) of the heat spot. The device for moving a light spot and a magneto-optical recording medium relative to each other may be a rotating driver for rotating a magneto-optical recording medium while controlling it at a desired rotational speed, or a device (scanner) for scanning a magneto-optical recording medium with a light spot. The irradiation of a rotating magneto-optical recording medium with a light beam produces in the recording layer of the medium a heat distribution based on the light intensity distribution. Since the rate of heat transfer is constant within the recording layer of a magneto-optical recording medium. If the rotational speed of the recording medium increases, the heat center of a heat spot is formed in back of the center of a light spot. FIG. 4 shows the relation between the rotational speed of a magneto-optical recording medium and the distance between the center of a light spot on the medium and the heat center of the associated heat spot. Accordingly, by rotating a magneto-optical recording medium at a desired speed while controlling the rotational speed by means of the rotating driver, it is possible to form the heat center of a heat spot outside the associated light spot. The magnetic field source and the lens are so positioned that the axis of the magnetic field generator of the magnetic field source coincides with the heat center of the heat spot which is formed outside the light spot.
A conventional recording/reproducing apparatus includes a magnetic field source having a magnetic field generator aligned with the axis of a lens so as not to intercept the light beam outgoing from the lens. Because the conventional apparatus applies a magnetic field to a wide region including a light spot, it is not possible to locally apply the field only to the highest temperature region of the heat spot which is based on the light intensity distribution.
In order to form outside a light spot a high temperature region of the heat spot which is produced on the basis of the light intensity distribution of the light spot, the present invention may include adjusting the diameter of the light spot and/or the light intensity, in addition to adjusting the relative speed between the light spot and the magneto-optical recording medium. This may be achieved by a high NA lens for making the light spot minute or a device for adjusting the intensity of the light beam radiated to the recording medium. For the material of the recording layer of the recording medium, the rate of heat transfer is constant. Consequently, if the light beam is radiated while the recording medium is rotated at a constant speed, the heat center of the heat spot which is produced on the basis of the heat intensity distribution is formed at a certain distance from the center of the light spot. Therefore, by using the high NA lens to make the light spot small, it is possible to form the heat center of the heat spot outside the light spot. The high NA lens may be a lens having a numerical aperture ranging between 0.7 and 0.95, or a pair of lenses consisting of a solid immersion lens (SIL) and a condensing lens for condensing a light beam to the SIL.
The recording/reproducing apparatus according to the invention may include a spindle motor or another rotating driver for rotating a magneto-optical recording medium at a desired speed. This apparatus may further include a controller for calculating a linear velocity from the rotational speed of the recording medium and the position of the magnetic field source over the medium, and controlling on the basis of the calculated linear velocity the distance between the center of the light spot and the region to which the magnetic field is applied. The controller may include a two-axis (biaxial) actuator which is used in, for example, an optical pickup. That is to say, it is possible to vary the position where the light beam is condensed by driving the actuator which changes the condensing lens while the magnetic field application region is maintained in a predetermined position. This makes it possible to control the distance between the center of the light spot and the field application region. Alternatively, as shown in FIG. 8, a galvanomirror 4 may be used to finely control the direction of the light beam incident on a pair of lenses 6. This, too, makes it possible to adjust the position where the light beam outgoing from the lens(es) is condensed. It is therefore possible to change the position of the center of the light spot.
In accordance with a sixth aspect of the invention, a magneto-optical head is provided for recording information on and/or reproducing information from a magneto-optical recording medium, the head being characterized in that it comprises:
a lens for condensing a light beam onto the recording medium; and
a magnetic field source for applying a magnetic field to the recording medium, the field source being narrower in the direction along the track of the medium than the diameter of a light spot formed on the medium by the lens;
the magnetic field source being formed at the bottom of the lens in such a manner that the axis of the magnetic field generated from the magnetic field source is off the axis of the lens.
The magneto-optical head according to the invention includes a magnetic field source, which may be a magnetic coil. The magnetic field source is formed at the bottom of a lens for condensing a light beam on a magneto-optical recording medium. The field source is so positioned that the center of the magnetic field generated from the source is off the optical axis of the lens. A high temperature region of the heat spot formed on a magneto-optical recording medium rotating and irradiated with a light beam is off the center of the light spot. The magnetic field generator of the field source can be positioned at the high temperature region formed off the center of the light spot. Therefore, if the field source is a magnetic head including a narrow magnetic field generator, the magnetic field can be applied only to the highest temperature region. This makes it possible to form a micro recording magnetic domain in the recording layer of the recording medium.
For example, as shown in FIG. 16, a conventional magneto-optical head 200 includes a lens (SIL) 10 and a magnetic coil (magnetic field source) 104 which are positioned coaxially with each other. Because the magnetic coil 104 is larger in diameter than the lens 10, a magnetic field is applied to the whole area of the light spot formed on a magneto-optical recording media by the lens 10. It is therefore impossible to apply the magnetic field only to the highest temperature region formed in the recording medium by light radiation. This does not allow to form a minute recording magnetic domain in the recording layer of the recording medium.
It is most preferable that the magnetic field source of the magneto-optical head according to the invention be so positioned at the bottom of the lens as not to intercept the optical path of the recording light beam or the reproducing light beam outgoing from the lens. If the field source is so positioned as to intercept the optical path, the source should preferably be so positioned that the rate of decrease in efficiency for light utilization is 50% or lower. Herein, the xe2x80x9cefficiency for light utilizationxe2x80x9d means the ratio of the quantity of reflected light to the quantity of incident light. The quantity of incident light can be measured by a front positioned monitor, which is positioned upstream with respect to a light beam incident on a magneto-optical recording medium. The quantity of reflected light can be measured by a signal detector detecting the light beam reflected by the recording medium.
The magnetic field source of this magneto-optical head may be a thin-film magnetic head, which can be lithographically made. The magnetic head may include a one-turn coil as shown in FIG. 11(D) or a two-turn coil as shown in FIG. 13(H). For example, it is possible to make a one-turn coil type thin-film magnetic head lithographically by laminating a substrate with copper films and a dielectric film in order in such a manner that patterns as shown on the right sides of FIGS. 11(A)-11(D). For example, it is possible to make a two-turn coil type thin-film magnetic head by laminating a substrate with copper films and dielectric films so as to form patterns as shown on the right sides of FIGS. 12(A)-12(D) and 13(E)-13(H). The views on the left sides of FIGS. 11-13 are typical sections as viewed in the direction X of the plan views on the right sides. It is preferable that the substrate of each of these magnetic heads be made of transparent material such as glass or plastic. It is preferable that the difference in refractive index between the substrate and the lens be small. The material of either the substrate or the lens should preferably be so selected that the difference in refractive index is 10% of or smaller than the refractive index of one of the substrate and the lens.
The magnetic field generator of the magnetic field source of this magneto-optical head should preferably have such a width that the region of a magneto-optical recording medium to which a magnetic field is applied is narrower than a light spot, and desirably as wide as or narrower than half the diameter of the spot, in the direction along the track of the medium. The size of the region of a magneto-optical recording medium to which a magnetic field is applied may be measured by a direct observation method with a magnetic force microscope, a magnetic field detecting element having a magneto-resistance effect, and/or the like, or by the indirect measurement of reproduced waveform with the recording/reproducing apparatus according to the invention and a magnetic super-resolution magneto-optical disc. This indirect measurement of reproduced waveform with the recording/reproducing apparatus and a magnetic super-resolution magneto-optical disc involves monitoring the reproduced waveform while varying the position of the magnetic head, and calculating the size of the magnetic field application region from the position of this head at a time when a reproduced signal is obtained and the position of the head at a time when no reproduced signal is obtained.
A magneto-optical recording medium has a recording track where information can be recorded. The recording track may be eccentric from the recording medium. In this case, while the magneto-optical head of a recording/reproducing apparatus is scanning the eccentric track of the recording medium set in the apparatus, the eccentricity may cause the portion of the track which is positioned just under the head to fluctuate in the direction across the track. It is therefore preferable that the magnetic field generator of the magnetic field source of a magneto-optical head be long enough in the direction across the recording track to be accommodated to the positional fluctuation of the track in the direction across the track due to the eccentricity. It is also preferable that the field generator be longer in the direction across the recording track than the diameter of a light spot formed on the track. If the magnetic field source includes a two-turn coil type thin-film magnetic head as stated earlier, it may be constructed as shown in FIG. 5. With reference to FIG. 5, a thin-film magnetic head 3 includes a magnetic field generator 3a, which has a length L in the direction across the track of a magneto-optical recording medium. The length L is larger than the diameter Sw of a light spot. The magnetic head 3 should preferably be so positioned that the center of the field generator 3a is nearly coaxial with the heat center formed on the recording medium.
The magneto-optical head according to the invention may include a controller for controlling, on the basis of a linear velocity of a magneto-optical recording medium, the distance between the center of the light spot formed on the medium by the lens and the center of the magnetic field generated from the magnetic field source. The control of the distance between the center of the light spot formed on the recording medium and the center of the magnetic field may involve controlling either the condensing position of the lens or the position of the magnetic field source relative to the lens.